In response to the demands of consumers who are driven both by ever-escalating fuel prices and the dire consequences of global warming, the automobile industry is slowly starting to embrace the need for ultra-low emission, high efficiency cars. While some within the industry are attempting to achieve these goals by engineering more efficient internal combustion engines, others are incorporating hybrid or all-electric drive trains into their vehicle line-ups. To meet consumer expectations, however, the automobile industry must not only achieve a greener drive train, but must do so while maintaining reasonable levels of performance, range, reliability, and cost.
In recent years, electric vehicles have proven to be not only environmentally friendly, but also capable of meeting, if not exceeding, consumer desires and expectations regarding performance. While early electric vehicles used DC motors in order to achieve the variable levels of speed and torque required to drive a vehicle, the advent of modern motor control systems have allowed AC motors to deliver the same level of performance while providing the many benefits associated with AC motors including small size, low cost, high reliability and low maintenance.
In addition to its many other beneficial characteristics, an electric motor is capable of providing a high starting torque and then rapidly modifying the developed torque as needed. For example, while an internal combustion engine (ICE) typically requires approximately 250 milliseconds to change its torque delivery, an electric traction motor is generally capable of changing its torque level in 10 milliseconds or less. As a result, electric motors hold the promise of greatly improved traction control over that achievable in an ICE-based vehicle. Unfortunately, as the wheel speed information used for traction control was developed based on the capabilities of ICE-based vehicles, this information is not updated at a high enough rate to take advantage of the capabilities of an electric traction motor.
A variety of approaches have been taken to try and utilize the capabilities of electric motors in the traction control system of an electric car. For example, U.S. Pat. No. 7,739,005 discloses an approach based on a conventional one motor per axle implementation that utilizes a low frequency controller (i.e., the first stage of a traction and stability control unit) in series with a high frequency controller (i.e., the second stage of the traction and stability control unit). The low frequency controller attempts to keep a constant slip rate between the tractive wheels and the vehicle speed while the high frequency controller attempts to limit sudden changes in motor speed. Although the disclosed controller is capable of sending torque to the axle that has the most traction, it unfortunately relies on a PI controller or another controller with a memory, either of which adds a delay in the system response. As a result of the added delay, the system behavior appears less natural and more intrusive to the driver.
Accordingly, what is needed is a traction control system that can take advantage of the electric motor's fast response while relying on low frequency wheel signals, e.g., the wheel signals supplied by an ABS controller. The controller should also minimize system delays, thereby insuring a natural feeling control system. The present invention provides such a control system.